An excimer laser has been conventionally developed for commercial use as a light source of a reducing projection and exposure device (hereinafter referred to as a stepper) for a semiconductor manufacturing apparatus. This is because an excimer laser enables extremely precise work, since the wavelength of the excimer laser is in an ultraviolet region and is short, thereby permitting the concentration of the light into a smaller area by means of an external optical element, such as a lens, when conducting the work.
Light oscillating from the excimer laser has various wavelength components, and the central wavelength varies. As a result, if the light is in the as-is status, an aberration will occur when the light passes through an external optical element, such as a lens, thereby reducing the accuracy of the work. For this reason, there is a widely used art of making a narrow band, in which an excimer laser is equipped with a wavelength selecting element, such as a grating, to narrow the spectral width of the laser oscillation wavelength, which is called a line width, and to stabilize the central wavelength as a central value of the oscillation wavelength.
FIG. 8 shows an example of a prior art disclosed in Japanese Laid-open Patent No. 3-214680. Laser gas is sealed in a chamber 1, and energy is supplied as a result of an electrical discharge in a discharge electrode 2 portion, whereby the laser beam 3 oscillates. The oscillating laser beam 3 exits through a rear window 5, the beam size thereof is widened while passing through a first prism 7 and a second prism 8, and then the laser beam 3 enters a grating 10. In the grating 10, an angle relative to the light path of the laser beam 3 is controlled by an actuator (not illustrated), and only by oscillating a predetermined wavelength, which is selected, the aforesaid narrow band is achieved. Here, a group of optical components, which are the first prism 7, the second prism 8, and the grating 10, is collectively called the narrow-band optics 12. The laser beam 3, with the wavelength being controlled by the narrow-band optics 12, passes through a front window 13 and a front mirror 14, which is a partial reflecting mirror, and part of the laser beam 3 exits to the right, as shown in FIG. 8.
Synthetic fused silica (SiO.sub.2) is generally used as the material of the optical components for the aforesaid excimer laser, as disclosed in Japanese Laid-open Patent No. 3-214680. This is because synthetic fused silica has a lower absorption rate with respect to ultraviolet light exiting from the excimer laser, and is easily processed. The kinds of materials, having high transmittivity for the ultraviolet light of 248 nm, which is the wavelength of a KrF excimer laser, or of 193 nm, which is the wavelength of an ArF excimer laser, are limited; and fluorides such as CaF.sub.2, MgF.sub.2, and LiF, are presently known as suitable materials instead of the synthetic fused silica.
However, at the present time, the art of manufacturing and processing these fluorides is not as mature, compared with that of synthetic fused silica. For this reason, it is difficult to produce optical material which is equivalent to synthetic fused silica in size, quantity, and optical properties and which can be manufactured. Therefore, in many cases, synthetic fused silica is used as the optical material. Especially in the KrF excimer laser, which has a comparatively long oscillation wavelength, the optical components are less deteriorated; therefore, optical components made of synthetic fused silica can endure for extended periods of use. Consequently, synthetic fused silica is generally used for the optical components for the KrF excimer laser.
However, there are the following disadvantages in using synthetic fused silica as the material for the optics for an excimer laser.
In order to manufacture semiconductors efficiently in large quantities, it is necessary to improve the manufacturing performance of a stepper; and there has been a demand to increase the power of a laser by increasing the laser oscillation pulse numbers per unit time (also called the repetition frequency). However, the energy density is high in the resonator of the laser, and moreover, a laser beam reciprocates in the resonator and passes through the optical components many times. For this reason, as the power of a laser becomes higher, the optical components, made of synthetic fused silica and used for the KrF excimer laser, are deteriorated as a result of even a little distortion or unevenness inside the material, which results in a disadvantage. Even a little deterioration of the optical components exerts a great influence on the quality of the oscillating laser beam, and makes it difficult to use the laser as a light source of a stepper.
Thus, the optical components of synthetic fused silica are insufficient in durability when the power of an excimer laser is increased, and a highly accurate control of the wavelength of an excimer laser is difficult when these optical components are used.
A situation in which the optical components are deteriorated by oscillating laser beams will be explained with reference to FIGS. 9 and 10. FIG. 9 shows a normal condition of the first prism 7 and the laser beam 3 passing through it. As the repetition frequency is increased, the inner portion of the prism 7 is distorted by the energy of the laser beam 3. When the distortion becomes greater, the surface of the prism 7 is shrunk; and as shown in FIG. 10, a distortion 15 occurs on the surface. Thus, the beam size of the laser beam 3 and the sectional shape of the beam are varied, the wave front of the laser beam 3 is disturbed, and the wavelength and the polarized light condition are not normal. This occurrence results in a disadvantage. As a result of the distortion 15, it may happen that a nonreflective coating, applied on the surface of the prism 7, comes off, and the laser beam 3 concentrates thereon, thereby breaking the prism 7.
FIGS. 11 and 12 show the distribution of narrow-band laser oscillation wavelengths before and after the deterioration of an optical component. When the optical component is deteriorated, as shown in FIG. 12, the central wavelength .lambda.c varies between .lambda.c and .lambda.c', for example, or the line width .DELTA..lambda. is increased. Therefore, such a laser is difficult to use as a light source for a stepper.